Fiber optic networks are known in the art. Fiber optic networks include a set of stations connected to a fiber optic transmission channel. A station typically includes a transmitter and receiver. A transmitter converts electrical pulses to optical pulses, which are applied to the fiber optic transmission channel. A receiver converts the optical pulses back to electrical pulses, which are then processed by another electronic device, such as a computer.
A set of combined optical pulses forms a transport packet. When a station connected to the fiber optic transmission channel has data to send to another station, it adds the address of the receiving station to the data to form a transport packet. The sending station then reads transport packets that are passing on the fiber optic transmission channel. If an empty transport packet is found, the sending station loads its address and data into the transport packet. Intermediate stations push the transport packet through the fiber optic transmission channel. If a station recognizes its address in a passing transport packet, it copies the data and adjusts the header associated with the address to indicate that the data was received. When the transport packet returns to the sending station, the sending station recognizes that the transport packet has been received at its destination and therefore converts the transport package to an empty transport package that can be used once again.
Double-ring fiber optic networks are used for increased reliability. In these networks, each station has two inputs and two outputs connected to two paths that operate in opposite directions. A double-ring optical network can bypass defective stations and send transport packets around a fiber break.
SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) is a communication standard used on double-ring fiber optic networks. The standard defines a number of basic transmission rates within SDH. The first of these is 155 Mbps, normally referred to as STM-1 (Synchronous Transport Module, level 1). SONET, on the other hand starts at a lower bit rate building block, STS-1 (Synchronous Transport Signal, level 1) at 51.84 Mbs. Higher line rates are integer multiples of these base rates.
SDH defines a data structure called a container. A container is a set of data. A path overhead envelope is attached to the container to form a virtual container. The path overhead envelope provides an address, a framing pulse, a check sum, error control, and other functions associated with the data.
FIG. 1 illustrates a virtual container 18A comprising a container 20A and a path overhead envelope 22A. As shown in FIG. 1, a number of virtual containers 18A, 18B, and 18C may be loaded into a payload envelope 24. Different combinations of virtual containers 18 can be used to fill up the payload envelope 24. For example, the payload envelope 24 may be an 87 byte by 9 byte data structure, which can be filled in a variety of ways. The process of loading containers 20 and attaching path overhead 22 is repeated at several levels, resulting in the "nesting" of smaller virtual containers 18 within larger ones. This process is repeated until the large size virtual container is filled, this is then loaded into the payload 24.
When the payload area 24 is full, additional control information is added to the frame to form the transport overhead envelope or fiber optic overhead channel 26. The fiber optic overhead channel 26 remains with the payload 24 as it traverses the fiber optic transmission channel between stations. Its purpose is to provide communication channels, user channels, protection switching, section performance, frame alignment, and other system functions.
The payload envelope 24 constitutes the bulk of the information that is transported over a fiber optic transmission channel. For example, the payload envelope 24 typically constitutes over approximately 95% of a transport packet's bandwidth, while the fiber optic overhead channel 26 constitutes less than approximately 5% of the transport packet's bandwidth.
Since the bulk of the information that is passed over a fiber optic transmission channel is in the payload envelope, most technology related to fiber optic transmission channels is directed toward the payload envelope. Accordingly, it would be highly desirable to use the fiber optic overhead channel in a more useful manner. In particular, it would be highly desirable to use the fiber optic overhead channel for fault-tolerant data transport. This would allow for a private data network that does not impact the revenue-bearing payload envelope.